LB#14

MAXIMIZING THE USE OF THE OVERHEAD RPOJECTOR 

In the booklet on 'Selecting Appropriate Presentation Techniques and Instructional Media', we saw that the overhead projector (OHP) is still probably the most useful and versatile visual aid that is available to the modern lecturer. It has long since replaced the traditional chalkboard as the main teaching aid provided in practically all classrooms and lecture theatres, and, although computer-driven data projectors are becoming increasingly widely used, their high cost means that it is highly unlikely that they will ever completely replace the OHP. It is therefore important that all teachers, lecturers and trainers should know how to make effective use of the OHP, so that they can exploit its potential to the full. This booklet has primarily been written to help new, inexperienced lecturers to acquire this basic knowledge and expertise, although it is hoped that more experienced staff will also be able to pick up some useful hints from it.

The booklet begins by discussing the main educational uses of the OHP, and highlighting its various strengths and weaknesses. It then offers some basic guidelines on how to use the OHP effectively, showing (for example) how to avoid common faults such as keystoning. Finally, it provides detailed guidance on how to design and produce OHP software.

How the OHP can be used in different instructional situations

Despite the increasing use that is being made of data projectors to display computer-generated materials to classes, the overhead projector is still probably the most useful visual aid available to anyone who wishes to carry out expository teaching of virtually any type. It is also extremely useful in many group-learning situations, both for presenting information to a group (eg in a seminar, or the briefing for an exercise of the game/simulation/case study type) and in enabling members of a group to display material that they themselves have produced (eg in a group project or case study). When linked with a laptop or personal computer via a liquid-crystal display tablet, the OHP simply performs these same functions in a more sophisticated way.

Some strengths of the OHP

The OHP has a number of definite advantages over most other methods of presenting visual information. A lecturer can, for example, use it in exactly the same way as a chalkboard or markerboard (for writing out notes, working through calculations and proofs, drawing graphic material, and so on) but with the great advantage of always facing the class, and thus being able to maintain eye contact with the learners. Such eye contact, which is, of course, impossible when a lecturer is writing on a chalkboard or markerboard, can play an extremely useful role in both expository and facilitative teaching, serving both as an outward non-verbal communication channel for the teacher and as a means of obtaining feedback from a class on how a session is going.

Another important advantage over the chalkboard or markerboard is that the OHP can also be used to show pre-prepared material, thus enabling teachers and lecturers to build up banks of notes, diagrams, tables etc. that can be used over and over again. When well planned and designed, such sets of overhead transparencies can often also provide all the cues and aides mémoire that are needed during a lesson, so that no conventional teaching notes are required. As we will see later, such material can be prepared using a wide variety of production methods (free-hand writing or drawing, typing, photocopying, desktop publishing, and so on) and can incorporate a wide range of presentation techniques (progressive disclosure, use of overlays, use of animation, etc.). Overhead transparencies are also relatively compact compared with some other types of visual aids (eg charts), and are therefore easy to store in suitable boxes, large envelopes, folders or files. When computer disk files are used, back-up copies should always be kept.

Compared with other projected aids, the OHP also has the great advantage that it does not require the room to be blacked out, thus allowing students to take notes; indeed it can be used in all but the very brightest light (eg direct sunlight), an advantage that tends to be lost when direct transmission of coloured electronic images is introduced, however. The OHP is also clean, quiet, and 'user friendly', requiring no technical skill or knowledge on the part of the operator apart from the ability to change the occasional lamp.

Disadvantages of the OHP include the fact that it requires a power supply, and needs a suitable flat (preferably white) surface on which to project its image. Also, unless this surface is inclined forward at the correct angle, the image will probably suffer from 'keystoning' (see Figure 1). Unlike chalkboards, OHP's do also require a certain amount of routine maintenance. They are also liable to break down occasionally (generally at extremely inconvenient times), so it is always advisable to have a spare bulb close at hand (most modern machines do in fact have a built-in spare bulb that can be brought into use at the turn of a knob.) A further disadvantage is that some lecturers find the glare from the OHP troublesome, although this can generally be overcome by attaching a suitably-positioned shade to the machine.

Apart from these possible 'hardware' difficulties, the main problems associated with the overhead projector stem from the fact that many users do not give sufficient thought to the production of their display material. In many cases, writing is too small or too untidy to be read easily (both in some cases), quite apart from the fact that it frequently extends beyond the visible area of the transparency. Teachers and lecturers tend to forget that the illuminated projection area in most overhead projectors is not the same size as the acetate sheets that are used to produce OHP transparencies. Most OHP projectors have platens that are roughly 10 inches (250mm) square, with the corners either truncated or rounded. Thus, neither the old square acetate sheets that used to be common nor the newer A4 sheets that have largely replaced them fit properly on the standard square platen. The square acetate sheets are bigger all round, while the A4 sheets are bigger in the vertical or horizontal direction, depending on whether they are used in 'portrait' or 'landscape' format. Some newer OHP projectors, it is true, actually have A4-sized platens, so, provided that A4 acetate sheets are used, the problem does nor arise with them. Most OHP's are still of the traditional 'square' variety, however.

Finally, teachers and lecturers tend, if anything, to overuse the overhead projector just because it is so convenient, employing it in situations where other forms of visual aid might, on occasions, be more effective.